ncurses: Add an ncurses-lib package

Set LIB_PACKAGE= in lib/ncurses/Makefile.inc so the ncurses libraries
go into the ncurses-lib subpackage rather than the base package. This
means applications that depend on ncurses can be installed without
needing to install the various utilities and manpages.

This removes a dependency from runtime to ncurses (it now depends on
ncurses-lib instead).

MFC after: 3 seconds
Reviewed by: kevans
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53060